Actuators for active vibration control can be realized utilizing hydraulic, pneumatic or electromagnetic implementation. Electromagnetic actuators commonly utilize moving armatures in configurations whose operating principles are characterized by large variations in magnetic air gap dimensions and consequent large variations in flux density in the air gap which, due to eddy currents and other harmful effects, limit efficiency and available stroke amplitude.
However a special category of electromagnetic actuator works on the principle of a loudspeaker voice coil moving in a small air gap of constant dimensions in which the flux density, typically from a permanent magnet, remains substantially constant, providing substantial design advantages.
The combination of voice coil type actuator and flat spring flexures has potential for providing the following advantages:
1. Maximized volumetric utilization with minimum mass requirements.
2. Capability to accommodate multiple actuator modules in parallel, where the stationary coils are mounted on a common base and the armatures containing the magnet structure(s) are connected to a common flexure pack so that high width to depth ratios are possible.
3. The active mass ratio is maximized because the shell is included in the armature structure.
4. Flexures allow longer stroke than coil springs.
5. A greater diversity of materials including composites is possible with flexures rather than coil springs.
6. Flexures allow much higher spring rates per unit is volume than coil springs.
7. The support flexure element accommodates the dynamic change in length of the working flexure element, locates the inertial mass, and transmits the inertial forces to the base in a minimum of space and minimum mass.
8. The coil assembly can benefit from improved thermal bonding to the base and forced cooling can be designed into the system.
When a voice coil type actuator is used in a small range of frequencies it is common practice to add springs to the arrangement in order for the armature to operate at or just below resonance. Resonance provides a means to create a force gain and reduces the power input required, but the amplitude of the armature does not change as a function of resonance. The amplitude of the armature is proportional to the output force of the actuator, not the input (coil) force.
Higher output force requires a longer actuator to accommodate longer stroke, or an actuator with a large diameter to accommodate additional inertial mass. Lower operating frequency also requires longer stroke, which requires very long springs, adding to the bulk of the actuator and reducing the ratio of active mass to total mass. The increased dimensions and mass of such actuators make them often unsuitable for applications such as helicopters